Our invention relates to a phase shift detector, and particularly to such a detector for detecting the phase shift in a tone transmitted in a radio communication system.
In some frequency modulation radio communication systems, a tone of audio frequency (usually below the voice frequencies) is transmitted to activate only a selected receiver or receivers, and thus provide privacy for the selected receiver or receivers, and prevent interference to other receivers during the transmission. This arrangement is sometimes referred to as a continuous tone controlled squelch system (hereafter CTCSS) or a channel guard system. If the transmitted radio frequency carrier is cut off when the transmission ends (by unkeying the microphone) then a burst or tail of noise will be heard at the activated receiver until the receiver noise squelch circuit takes over and mutes the receiver. In order that this tail of noise be blocked or eliminated, some frequency modulation communication systems use an arrangement that continues transmission of the carrier for a short period (such as 175 milliseconds) after the microphone is unkeyed. During this period while the carrier continues to be transmitted, the phase of the tone is shifted. Detection of this phase shift is necessary in order to deactivate the receiver during the short period that the carrier is still transmitted, so that no noise will be heard when the carrier is finally terminated.
Accordingly, a primary object of our invention is to provide a new and improved tone phase shift detector.
Another object of our invention is to provide a new and improved phase shift detector for use with CTCSS radio receivers.
Phase shift detectors for CTCSS radio receivers have been provided. Typically, such detectors used an electronic circuit tuned to the frequency of the tone to which the receiver was to respond. While the tone was received, the tuned circuit produced an output whose magnitude depended upon how close the transmitted tone frequency and the resonant frequency of the tuned circuit were to each other. Typically, the two frequencies could differ by a substantial amount, either as a result of the tone frequency being incorrect, or as a result of the tuned circuit resonant frequency being incorrect. If the relative difference in the two frequencies was as much as only 0.5 percent, detection of the phase shift became difficult or unreliable.
Accordingly, another object of our invention is to provide a tone phase shift detector that provides an improved or good indication of phase shift, despite relatively large differences in the frequency of the transmitted tone and the frequency to which the electronic circuit is tuned.
Another problem in detecting a tone phase shift results from the fact that the communication medium, or noise, or some other factor may cause voltages or signals to be received that could appear as though the transmitted tone has undergone a phase shift.
Accordingly, another object of our invention is to provide a new and improved tone phase shift detector that is relatively immune from noise and other factors present in a communication system.
Prior art U.S. Pat. No. 3,962,645 shows a circuit for detecting tone frequencies in a CTCSS receiver.
Accordingly, a fairly specific object of our invention is to provide a tone phase shift detector for use with the circuit shown in U.S. Pat. No. 3,962,645.